Much has been posted in different threads on these subjects. I would simply like to share the experience a group of four travelers on different records and different fares using different UG instruments had the other day. Everything worked out as it was supposed to in the end, but it took some work, and DL's IT still did not do its job perfectly.

The scenario: a family of four traveling from Europe to LAX connecting in JFK. Three different records, just happened to be done that way for various reasons. Effectively, three independent groups of travelers:

Records:
A DM + 1 no status on M;
1 no status on M;
and a GM on Q.

Q obviously not upgradable on the TATL leg; others requested upgrades with miles, and the DM with a SWU. That part went smoothly, no surprises. The last UG with miles was waitlisted and cleared at the gate (so, yes, it is possible to go on the gate list not only with SWUs but also with mileage UGs).

The unusual things happened on the JFK-LAX leg (NB: I checked all records on delta.com; all three were coded correctly, with OP upgrades waitlisted, plus comp for GM and DM passengers).

The DM + 1 party were 9 and 10 on the list (i.e., coded as comp rather than OP UG).

The GM on Q was #1 on the list (surprisingly).

The non-status on M was not on the list at all.

Spoke to an agent who did not know how to help (did not know about different upgrade priorities, coding, was clueless about UP2B; from my many experiences, a very common problem at the JFK T3 arrivals area). Then asked for a Red Coat, who got someone more knowledgeable who corrected the record by checking us in again). In the end, the UG WL priority was:

GM on Q using miles, #1
DM + 1 on M using miles/SWU, #2 and #3
no status on M using miles, #22.

Asked to check on the last passenger, told it did not seem right. Got him unchecked, then checked in again, then he ended up #4 on the list, with the same UP2B as everyone else.

The DM on M should have been higher on the list than the GM on Q, but it did not matter for us (all got upgraded).

So, no complaints (we had enough time between the flights, but it would not have been doable if we did not -- it took 50 minutes to fix), mostly just a note to share our experience and to confirm that (a) mileage upgrades transfer to the gate even if they don't clear in advance; (b) they get the same priority, UP2B, as would SWUs; and (c) DL's IT is still screwy and one would be well advised to check the accuracy of the record with a knowledgeable agent.